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I'm about to tackle a DIY subwoofer build. I'm totally comfortable with the woodworking but want to make sure I understand the acoustics, so I have a few questions: - I've been playing around with a subwoofer box designer from http://www.ajdesigne...aker/index.php. I input all the parameters of my driver, but I want to make sure I understand Qtc, which isn't on the driver spec sheets. I can pick the value that I want for that, right? And depending on what I pick, I will get a different optimum volume and a different rolloff response? - If I want to build a dual-driver opposed design (sealed), do I just double the sealed volume I compute for a single driver? or is it more complicated than that. - Once I have my desired volume, should I add any loose fill or padding, or is that only for enclosures that are undersized? - I'm toying with the idea of a non-rectangular enclosure (maybe a trapezoid), mainly for aesthetic reasons. Is that going to help or hurt the performance? Or does it not matter as long as the volume is correct? Thanks for the help!

Should not be that hard. You can actually get the suggested volume from most driver manufacturers and just build from there. Don;t forget the factor in the space that the driver takes up inside the box, and any bracing. I built an enclosure a while back, about 20 years ago. I bought a pair of 10" kicker subs and one phone call and we came up with something like 3.25 to 3.5 Qft sealed for the freq range I was looking for. I used 3/4" MDF, added a center brace and corner braces inside and filled with aquarium filter fiber. Worked great. It's easy when you can work with basic 6 sided shapes. About 4 years ago I made a fiberglass enclosure for a car. I custom fitted the enclosure to fit neatly in the hatch area. Totally odd shape and the only way O was able to figure out the volume of what I made was to fill it with water and measure that. Turned out I had about 2/3 of a foot in there. I ended up finding a 10" driver that would work ok in that small space. Again made some phone calls before a tech at Boston Acoustic determined that they have a shallow fit subwoofer that would sound great and reach down to about 36 hz in a small space. Yea, these have been car audio drivers, but they seem to work perfectly in the home and construction seems really, really good. Keep in mind that car audio subs are 4 ohm or lower.

I want to make sure I understand Qtc, which isn't on the driver spec sheets.

Qtc is a combination of the driver's Qts and the enclosure it is installed in. Years ago, someone decided that .707 is the optimum Qtc that gives the lowest Fs and the best roll-off. I like a Qtc around .500 but to get that, you have to start with a driver that has a low Qts. Your Qtc will never be lower than your driver's Qts.

If I want to build a dual-driver opposed design (sealed), do I just double the sealed volume I compute for a single driver? or is it more complicated than that.

It is that simple.

Once I have my desired volume, should I add any loose fill or padding, or is that only for enclosures that are undersized?

I always add it. For smaller enclosures, I just use loose ployfil from Wal-Mart. For larger ones, the polyfil pillows work great. One pillow per 1.5 to 2 cubic feet of space seems to work.

I'm toying with the idea of a non-rectangular enclosure (maybe a trapezoid), mainly for aesthetic reasons. Is that going to help or hurt the performance? Or does it not matter as long as the volume is correct?

Volume of the enclosure is all that matters. The wavelengths of the bass frequencies are longer than most boxes you can build.

That is not a correct statement. Go to Parts Express and look at the Dayton line of subs. The DVC, Titanic and RS subs are all marketed towards the home audio crowd and they are 4 ohm. It's better to remove the marketing from sub building and focus on the Thiele/Small parameters. I run TC Sounds subs in my home and they were marketed to the car audio crowd in the beginning. Now TC Sounds just sells subs. You pick the sub that best fits your application and goals.

Thanks for the help. While I have you, how about a couple more questions: - Built-in plate amp vs standalone amp. Any reason to pick one over the other? - I'm thinking of going with Dayton 15" drivers, maybe the "Reference HF" or "Ultimax DVC". Would those make a good choice? Is there something else I should be looking at in that price range? ($300-$400 for two drivers). There are *so many* options even once I've decided on 15", I've mainly been looking at Dayton because it just narrows down the choices.

Plate amps are great when you are building a ported sub since 99% of them come with a subsonic filter. Pro amps have the best power to dollar ratio. Plus if you go with a DSP model (Crown or Behringer) then you can easily tame room nodes without adding additional equipment. Stereo Integrity's home theater subs have over 50% more excursion and they are still under your budget. The 15's are in stock now. The 18's are on pre-order for a reduced price. Or get 6 Infinity Reference 1262Ws. Put 3 in each cabinet. Wire each driver as 8 ohms and wire all 3 for a final load of about 3.2 ohms. A Behringer 4000 would be pushing about 200w per driver.

OK, now you've inspired more questions. If I'm trying to pick a driver, should I *only* be paying attention to the specs, like excursion and Qts? Are those Stereo Integrity drivers automatically better than a Dayton Audio because you can see it from the numbers? Or is there anything qualitative about subwoofers that isn't well represented by spec sheets?

Or is there anything qualitative about subwoofers that isn't well represented by spec sheets?

Yes. Most subs don't include their inductance numbers. This gives you an idea on their sound quality. The more copper in the motor structure, the lower the inductance and the better they sound without additional equalization. That's not saying that a high inductance will sound bad. Mine have a number that is through the roof but with a little EQ, they sound great. I also get the benefit of more excursion (xmax is the excursion parameter). Here are my subs - http://wayback.archi....com/tc3000.htm I like the SI subs because I like working with the smaller companies. If you have an issue, you can send them an e-mail and Nick (the owner) will respond usually.

OK, now you've inspired more questions. If I'm trying to pick a driver, should I *only* be paying attention to the specs, like excursion and Qts? Are those Stereo Integrity drivers automatically better than a Dayton Audio because you can see it from the numbers? Or is there anything qualitative about subwoofers that isn't well represented by spec sheets?

There's a *lot* that the t/s specs won't tell you about the driver's performance. Remember, they are all small signal. I can't tell you how many times we found the "weaker" driver (based solely on t/s) performed better in real world measurements(compression, cea-2010 bursts, etc) One of the root problems here is inconsistency in t/s measurement methods/equipment. Ideally, you can purchase 20 different drivers and match several enclosures/amps to each to see what works. That's not practical for most though. The next best scenario is seeing repeatable measurements performed by someone like Josh (data bass) on a driver that may interest you. I'd try to avoid depending only on a t/s spec sheet if possible. Also, try not to get too hung up on t/s even if they have been verified. When you are looking at 2 or 3 really good drivers(like the dayton and SI here) the DIY will work really really well either way if you nail the enclosure and amp side. The average DIYer might only get 80% from the driver but that's still going to be better than most/all like priced retail woofs.. Tom V.